Electroplating, the application of a metal to the surface of a material by electrolysis, is typically used in the fabrication of thin film devices to produce relatively thick layers of metal. The electroplating process involves applying an electrical energy source to vary the electropotential of a substrate workpiece in the presence of an electroplating solution.
One of the problems in the use of electroplating is the difficulty in maintaining a suitable uniformity in thickness and composition of deposited films. In the fabrication of thin film devices, such as magnetic core elements of thin film heads, advantages are achieved by fabricating the devices using a Permalloy material. The chemical description of a Permalloy material is NiFe. The iron content (Fe) of Permalloy is typically precisely controlled, in one example to 18 percent by weight. The process of electroplating a large alumina wafer, for example a 6 inch diameter circular wafer, typically dictates usage of an electroplating procedure resulting in a substantially uniform iron (Fe) concentration across the entire wafer. FIG. 1 is a pictorial diagram of a circular wafer 100, such as a six inch circular wafer, including a plurality of devices 110 which are fabricated using thin-film processes.
The nature of electrolytic plating of a substrate wafer is that the current density applied to the substrate is substantially greater at the periphery of the wafer than near the wafer center. FIG. 2 illustrates various regions of the circular wafer 100 having different current densities. A low current-density region 112 exists at the center of the wafer 100. A medium current-density region 114 surrounds the low current-density region 112. A high current-density region 116 encloses the medium current density region 114. These regions are for illustrative purposes since the current density varies continuously from the center of the wafer 100 to the periphery of the wafer 100. This higher current density results in an increased deposition rate of plated film at the periphery of the wafer. It follows that the film plating thickness is increased at the peripheral edges of the wafer as compared to the wafer interior. In the case of deposition of permalloy, an alloy of nickel and iron, the concentration of iron (Fe) is higher at the periphery of the wafer due to the higher current density in the peripheral region.
Problems of nonuniform deposition of thin films are accentuated for large wafers. For example, in large diameter circular wafers having a diameter of 6 inches or greater, deposition composition and thickness control is a very difficult problem.
The thin film thickness differences between the interior and periphery of a wafer cause practical difficulties in plating of specific structures. Illustratively, one type of selected electroplated structure, for example plating of a large bottom pole 310 having a length of 200 .mu.m and a width of 10 .mu.m shown in FIG. 3, may have an iron content of 18% in the center region of the wafer, and 20% iron content in the periphery of the wafer. (Note that FIG. 3 is not drawn to scale.) Thus, the electroplating current distribution varies significantly in different regions of the wafer, resulting in a substantial variation in iron composition. Such a large variability in composition is not acceptable.
What is needed is a simple and cost-effective electroplating technique and system which maintains a substantially uniform metal composition and thickness while constructing thin film structures composed of a large number of thin film layers.